1. Field of the Invention
The present invention relates to a camera having a lens installed thereon and, more particularly, to the determination of an aperture value for an internal aperture inside the camera in accordance with the aperture value of an aperture inside the installed lens.
2. Description of the Related Art
Many conventional electronic silver salt cameras are designed to have a photographic lens installed thereon. Such cameras have an image pickup element positioned in the film plane. The photographic lens forms an optical image on the image pickup element.
Generally, only part of the optical image formed by the photographic lens is picked up by the image pickup element since the photoreceptor plane of the image pickup element is generally small in comparison with a typical film size of 24.times.36 mm. Therefore, a conventional type of electronic silver salt camera positions a reduction optical system between the photographic lens and the image pickup element. In this manner, the optical image formed by the photographic lens is reduced to the size of the photoreceptor plane of the image pickup element. Such a conventional silver salt camera is disclosed, for example, in Japanese Laid-Open Patent Publication No. HEI 6-169428.
FIG. 1 is a diagram illustrating such a conventional silver salt camera. Referring now to FIG. 1, a photographic lens 51 for a silver salt camera is installed on an electronic camera body 50. A lens aperture 52 is positioned inside photographic lens 51. A quick return mirror 53 is positioned at an angle to the optical axis I of photographic lens 51, and a half-mirror 54 is positioned at an angle in the reflection direction 60 of quick return mirror 53. A photometric unit 55 is positioned in the permeation direction 62 of half-mirror 54, and a viewfinder 56 is positioned in the reflection direction 64 of half-mirror 54. A reduction optical system 57, an internal aperture 58, and a CCD element 59 are positioned behind quick return mirror 53 along the optical axis I of photographic lens 51. The camera has a release button (not illustrated) having a half-push position and a full-push position. Various photographic processes, such as rangefinding and photometry, are performed when the photographer pushes the release button to the half-push position. An exposure operation is performed when the photographer pushes the release button to the full-push position.
In a camera as illustrated in FIG. 1, when lens aperture 52 in photographic lens 51 is held in an open condition, the pushing of the release button to the half-push position causes photometric unit 55 to perform photometry on light from the photographic subject (not illustrated) passing through lens aperture 52.
Here, when the apex value of the subject brightness is "Bv," and the apex value of the open aperture value of lens aperture 52 is "AvO," the apex value Lv of the photometric value from the photometric unit 55 is found from the following Equation 1.
Equation 1: EQU Lv=Bv-AvO
Alternatively, when the apex value of the correct exposure value is "Ev," the apex value of the aperture value of internal aperture 58 is "Av," the apex value of the exposure time is "Tv," and the apex value of the sensitivity of the CCD element 59 is "Sv," the correct exposure Ev can be found from the following Equation 2.
Equation 2: EQU Ev=Av+Tv=Bv+Sv
The following Equation 3 is obtained when Equation 1 is substituted into Equation 2.
Equation 3: EQU Ev=Av+Tv=(Lv+AvO)+Sv
Therefore, the right side of Equation 3, above, is computed from the apex value Lv determined by photometric unit 55 and the open aperture value AvO received from photographic lens 52. In this manner, the correct exposure value Ev is determined.
After the correct exposure value Ev is determined, and the photographer pushes the release button to the full-push position, the aperture value Av of internal aperture 58 and the exposure time Tv of CCD element 59 are adjusted to match the correct exposure value Ev, and the subject is photographed.
In this type of electronic camera, the incident luminous flux 90 from photographic lens 51 passes through two stages (a first stage being a pass through lens aperture 52 and a second stage being a pass through internal aperture 58) before reaching CCD element 59. Therefore, the amount of luminous flux is limited by the diameter of lens aperture 52. As a result, if lens aperture 52 is opened to a maximum diameter to allow an upper limit of luminous flux to pass therethrough, the illumination of the image plane of CCD element 59 cannot be increased any further. Thus, increasing the diameter of internal aperture 58 will not increase the illumination of the image plane beyond the upper limit allowed by lens aperture 52.
Because this upper limit on the luminous flux varies with the photographic lens being used, it is necessary for the photographer to perform an exposure setting operation while taking into account the aperture value control range for each photographic lens used. For this reason, the exposure setting operation is difficult and complicated.